The use of bulky ligand metallocene-type catalyst systems in polymerization processes to produce a diverse array of new polymers for use in a wide variety of applications and products is well known in the art. Typical bulky ligand metallocene-type compounds are generally described as containing one or more ligands capable of η-5 bonding to the transition metal atom, usually, cyclopentadienyl derived ligands or moieties, in combination with a transition metal selected from Group 4, 5 or 6 or from the lanthanide and actinide series of the Periodic Table of Elements. Exemplary of the development of these and other metallocene-type catalyst compounds and catalyst systems are described in U.S. Pat. Nos. 5,017,714, 5,055,438, 5,096,867, 5,198,401, 5,229,478, 5,264,405, 5,278,119, 5,324,800, 5,384,299, 5,408,017, 5,491,207 and 5,621,126 all of which are herein fully incorporated by reference.
It is also well known that these bulky ligand metallocene-type catalyst systems have a tendency toward fouling and/or sheeting, particularly when they are supported on a carrier, and especially when used in a gas or slurry polymerization process.
For example, in a continuous slurry process fouling on the walls of the reactor, which acts as heat transfer surface, can result in many problems. Poor heat transfer during polymerization can result in polymer particles adhering to the walls of the reactor, where they can continue to polymerize. This can be detrimental to the process and can result in premature reactor shutdown. Also, depending upon the reactor conditions, some of the polymer may dissolve in the reactor diluent and redeposit on for example the metal heat exchanger surfaces.
In a continuous gas phase process for example, a continuous recycle stream is employed. The recycle stream composition is heated by the heat of polymerization, and in another part of the cycle, heat is removed by a cooling system external to the reactor. Fouling in a continuous gas phase process can lead to the ineffective operation of various reactor systems. For example, the cooling system, temperature probes and the distributor plate, which are often employed in a gas phase fluidized bed polymerization process can be affected. These upsets can lead to an early reactor shutdown.
Another major problem associated primarily with fluid bed gas phase operation involves “sheeting” in the reactor. This is particularly problematic with bulky ligand metallocene-type catalysts because of their very high activity on a per gram of metal basis that often results in the generation of extreme heat local to the growing polymer particle. Also, the polymer produced with these bulky ligand metallocene-type catalysts are very tough, making the molten sheet that may form in the reactor difficult to break-up and remove from the reactor. Another problem associated with using a supported bulky ligand metallocene-type catalysts is that often there is a partial or complete pluggage of the catalyst delivery tube used to introduce the supported catalyst into the reactor. This pluggage phenomenon is particularly a problem when using very high activity, a high comonomer incorporating supported bulky ligand metallocene-type catalyst system.
As a result of the reactor operability issues associated with using supported bulky ligand metallocene-type catalysts and catalyst systems various techniques have been developed that are said to result in improved operability.
For example, various supporting procedures or methods for producing a metallocene-type catalyst system with reduced tendencies for fouling and better operability have been discussed in the art. U.S. Pat. No. 5,283,218 is directed towards the prepolymerization of a metallocene catalyst. U.S. Pat. No. 5,332,706 and 5,473,028 have resorted to a particular technique for forming a catalyst by “incipient impregnation”. U.S. Pat. Nos. 5,427,991 and 5,643,847 describe the chemical bonding of non-coordinating anionic activators to supports. U.S. Pat. No. 5,492,975 discusses polymer bound metallocene-type catalyst systems. U.S. Pat. No. 5,661,095 discusses supporting a metallocene-type catalyst on a copolymer of an olefin and an unsaturated silane. PCT publication WO 97/106186 published Feb. 20, 1997 teaches removing inorganic and organic impurities after formation of the metallocene-type catalyst itself. PCT publication WO 97/15602 published May 1, 1997 discusses readily supportable metal complexes. PCT publication WO 97/27224 published Jul. 31, 1997 relates to forming a supported transition metal compound in the presence of an unsaturated organic compound having at least one terminal double bond.
Others have discussed different process modifications for improving operability with metallocene-type catalysts and conventional Ziegler-Natta catalysts. For example, PCT publication WO 97/14721 published Apr. 24, 1997 discusses the suppression of fines that can cause sheeting by adding an inert hydrocarbon to the reactor. U.S. Pat. No. 5,627,243 discusses a new type of distributor plate for use in fluidized bed gas phase reactors. PCr publication WO 96/08520 discusses avoiding the introduction of a scavenger into the reactor. U.S. Pat. No. 5,461,123 discusses using sound waves to reduce sheeting. U.S. Pat. No. 5,066,736 and EP-A1 0 549 252 discuss the introduction of an activity retarder to the reactor to reduce agglomerates. U.S. Pat. No. 5,610,244 relates to feeding make-up monomer directly into the reactor above the bed to avoid fouling and improve polymer quality. U.S. Pat. No. 5,126,414 discusses including an oligomer removal system for reducing distributor plate fouling and providing for polymers free of gels. EP 0 453 116 A1 published Oct. 23, 1991 discusses the introduction of antistatic agents to the reactor for reducing the amount of sheets and agglomerates. U.S. Pat. No. 4,012,574 discusses adding a surface-active compound, a perfluorocarbon group to the reactor to reduce fouling. WO 96/11961 published Apr. 26, 1996 discusses as a component of a supported catalyst system an antistatic agent for reducing fouling and sheeting in a gas, slurry or liquid pool polymerization process. U.S. Pat. No. 5,026,795 discusses the addition of an antistatic agent with a liquid carrier to the polymerization zone in the reactor. U.S. Pat. No. 5,410,002 discusses using a conventional Ziegler-Natta titanium/magnesium supported catalyst system where a selection of antistatic agents are added to directly to the reactor to reduce fouling. U.S. Pat. Nos. 5,034,480 and 5,034,481 discuss a reaction product of a conventional Ziegler-Natta titanium catalyst with an antistat to product ultrahigh molecular weight ethylene polymers.
There are various other known methods for improving operability including coating the polymerization equipment, injecting various agents into the reactor, controlling the polymerization rate, particularly on start-up, and reconfiguring the reactor design.
While all these possible solutions might reduce fouling or sheeting somewhat, some are expensive to employ and/or may not reduce both fouling and sheeting to a level sufficient for the successful operation of a continuous process, particularly in a commercial or large-scale process with supported bulky ligand metallocene-type catalysts.
PCT Publication WO 97/46599 published Dec. 11, 1997 relates to the use of soluble metallocene catalysts in a gas phase process utilizing soluble metallocene catalysts that are fed into a lean zone in a polymerization reactor to produce stereoregular polymers. This PCT publication generally mentions that the catalyst feedstream can contain antifoulants or antistatic agents such as ATMER 163 (available from ICI Specialty Chemicals, Baltimore, Md.).
EP-A2-811 638 discusses using a metallocene catalyst and an activating cocatalyst in a polymerization process in the presence of a nitrogen containing antistatic agent. This European publication mentions various methods for introducing the antistatic agent, most preferably the antistatic agent is sprayed into the fluidized bed of the reactor. Another method generally discussed is the adding of an antistatic agent with the supported or liquid catalyst stream so long as the catalysts are not severely affected or poisoned by the antistatic agent. In the examples the supported catalysts were slurried in mineral oil prior to being introduced to the reactor and in the examples using the unsupported catalysts, the antistatic agent was introduced directly to the reactor.
Thus, it would be advantageous to have a polymerization process capable of operating continuously with enhanced reactor operability while at the same time producing polymers having improved physical properties. It would also be highly advantageous to have a continuously operating polymerization process having more stable catalyst productivities and reduced fouling/sheeting tendencies and increased duration of operation.